Cellular mobile radiotelephone systems contemplate telecommunication channel reuse pattern iteration to enable the coverage of an extensive geographical area for including and linking together plural large metropolitan areas with a single continuous pattern iteration. One system for an urban area, and characterized as the "advanced mobile phone service," is described in Vol. 58, No. 1, of the Bell System Technical Journal (BSTJ) issue of January, 1979.
Switching technology and the economics of circuit length impose limitations on the number of cell antenna sites and the extent of a geographical area which may be covered in a single mobile service area (MSA) controlled from a single mobile telecommunication switching office (MTSO). Just as mobile radiotelephone units pass from cell to cell in a single MSA, there will be times when mobile units will need to pass from MSA to MSA during the course of a single call in progress. A similar situation has been encountered in route-based systems extending along a railway or a highway route. One such system is shown in the U.S. Pat. No. 2,896,072 to A. E. Bachelet et al. In the Bachelet patent, trunks are included between control points connected to different central offices and are utilized in response to teletypewriter circuit signals between control circuits to enable an operator to transfer a call from one area to another by way of extended trunks. However, operator invtervention is required, and there is little or no occasion in a route-based sytem for a mobile unit to pass through a given mobile service area more than once during the course of a single call.
In "High-Capacity Mobile Telephone System Technical Report," filed Dec. 20, 1971, with the Federal Communication Commission under docket 18262, the movement of mobile units among different areas is considered. In land systems, movement among metropolitan coverage areas is mentioned at pages 1-2 through 1-4, and 3-6 through 3-10; and in air systems, movement among regions controlled by different switching offices is mentioned at pages 3-48 through 3-52. It was there considered that no call handoff would be required between metropolitan coverage areas of a land system but that mobile switching offices within one such area could be interconnected on call origination. For an air-ground system, it was considered tht up to one handoff of a call between air mobile switching offices could be accomplished under the control of an air data center.
In none of the prior art was consideration directed to the possibility of excessive interoffice trunk accumulation in a single call connection because of either topography in a boundary region between service areas or a particular route followed by a mobile unit in such a region. This type of accumulation leads to reduced received signal quality and inefficient utilization of interoffice trunk resources.